is plural of "Opus." It came about as being a collection of individual, er, opera in the form of arias, sinfoniae, etc., each styled an "opus" in and of itself, the whole packaged as a singular over-all entity. See also "datum," "agendum," et ceterus paribus.

The Brits do the same thing in reverse when they pluralize the verb with singular nouns (such as "the Army have just won the war," etc.) that are taken in a collective sense. I think Brits are more comfortable, as well, with using plural personal pronouns as gender-neutral substitutions for gender-specific singular pronouns, which is a practice common from Shakespeare to Jane Austin, up to the time that nineteenth-century grammarians aping the formalities of Latin grammar made this practice an infraction. Know your history and set your grammar free.